feat(card): configurable start of week (#25)

* feat(card): configurable start of week for heatmap + weekday cards

New -start-of-week CLI flag (and start_of_week action input) rotates the
contribution-heatmap rows and the productive-weekday bars so users whose
calendars start on Monday (or any other day) get matching output. Default
stays Sunday to preserve existing renders.

* docs: note start-of-week in design-guidelines, codebase-summary, deployment-guide

- design-guidelines: heatmap row order + productive-weekday bar order now derive from Profile.WeekStart
- codebase-summary: list new weekday_start_test.go cases + TestParseWeekday
- deployment-guide: mention start_of_week as an optional action input in the workflow template

docs/deployment-guide.md

@@ -30,6 +30,7 @@ jobs:
           token: ${{ secrets.GHSTATS_TOKEN }}
           themes: dracula,github_dark,tokyonight
           tz: Asia/Saigon
+          # start_of_week: monday   # optional; default sunday — rotates heatmap rows + weekday bars
           include_forks: "false"
           include_private: "false"
           commit_changes: "true"